Orthogonal frequency division multiplexing (OFDM) signals each have a guard interval (GI), which is a kind of cyclic prefixes (hereinafter, referred to simply as “CP”), at the head of each OFDM symbol.
The guard interval is a copy of a tail portion of an effective symbol and is provided to appropriately acquire data from a received OFDM symbol even if a receiving apparatus receives the OFDM symbol delayed from a proper timing due to transmission channel delay or even if a time waveform at a boundary between OFDM symbols is deformed due to multipath fading.
Conventionally, in order to extract frame timing and symbol timing of a received signal, used is a correlation operation to measure the degree of similarity between waveforms. In this case, the correlation operation between a received signal and patterns (standard signals) having excellent autocorrelation characteristics is performed so as to determine a position at which the square of the obtained correlation value is a maximum as the frame timing and acquire the symbol timing from the frame timing. Calculation of the frame timing using such a correlation method is disclosed in Japanese Patent Application Laid-Open Gazette No. 2008-153927.
Herein, the standard signals are signals generated by shifting a head position of a data part of a known signal at predetermined intervals (symbol intervals), and positions in the GI, which are obtained when the GI is equally divided by the predetermined interval, are referred to as cut positions. Since a standard signal that coincides with a cut position is determined as a reference signal, these positions are referred to as “cut positions”.
The cut positions are set at intervals of, for example, eight symbols by equally dividing the GI consisting of 64 symbols by eight. FIG. 11 shows respective frames of standard signals that are thus set and a frame of a received signal having a GI that is equally divided into eight parts. The cut positions are labeled 8/8, 7/8, 6/8, 5/8, 4/8, 3/8, 2/8, 1/8, and 0/8, respectively, in order from the head of the GI provided at the head of the data part, and the head of the data part is 0/8.
Meanwhile, the standard signals are generated by shifting the position of the head of the data part at intervals of eight symbols. A 0/8 standard signal has the highest correlation with the received signal in a case where the received signal is not shifted.
As shown in FIG. 11, the standard signals are each generated by adding a copy of data of 64 symbols at a tail of the data part of the received signal, which is an original of the GI of the received signal, to the head of the data part as a copied data tail portion.
Further, a 1/8 standard signal has the highest correlation with the received signal in a case where the received signal is shifted by 8 symbols, and a 2/8 standard signal has the highest correlation with the received signal in a case where the received signal is shifted by 16 symbols. In this way, eight standard signals from 0/8 to 7/8 are prepared on the basis of the received signal and are stored in a form of a table in a memory or the like. Setting the number of standard signals to eight in this way is to limit the number of correlation operations to be performed. Therefore, the actual shift of the received signal does not always coincide with the prepared standard signal, and depending on the degree of shift of the received signal, there may be a case where no position at which the correlation value is maximum can be acquired.
For example, when the received signal is shifted so that the position indicated by the arrow A in FIG. 11 actually becomes the position where the correlation value is maximum, the actual cut position cannot be determined since no standard signal corresponding to that position is prepared. In this case, either the 1/8 position or the 2/8 position after or before the arrow A is determined as the cut position.